travis: check spelling of binaries and manpages, use docker

We now build a docker base container based on debian sid (where the very
latest packages are available). That base container is updated once a
month, or whenever travis-build.Dockerfile or debian/control change, but
re-used for subsequent travis runs. While the initial build might take
up to 15 minutes, subsequent builds typically run in a minute or two.

All the different steps that we run on travis are now factored into
separate scripts in the travis/ directory.

Switching to docker should also help with issue #2174.

src/config_parser.c

@@ -910,7 +910,7 @@ bool parse_file(const char *f, bool use_nagbar) {
     FREE(bufcopy);
 
     /* Then, allocate a new buffer and copy the file over to the new one,
-     * but replace occurences of our variables */
+     * but replace occurrences of our variables */
     char *walk = buf, *destwalk;
     char *new = smalloc(stbuf.st_size + extra_bytes + 1);
     destwalk = new;